1. Field of the invention
This invention relates to a joint construction of structural pipes.
2. Description of the prior art:
A pipe joint construction 3 in which, as shown in FIG. 1, branch pipes 2, 2 are jointed or welded to a main pipe 1 is of a geometrically complex shape, thus there tends to arise marked stress concentration.
In case the main pipe 1 is compressed under loads F.sub.1, F.sub.2, the branch pipe 2 is compressed under a load F.sub.3, and the other branch pipe 2 is under tension by a load F.sub.4, then stress concentrations occur at points C.sub.1, C.sub.2, C.sub.3 on the weld lines of the main pipe 1 and the branch pipe 2. At this time, the maximum stress is created at the point C.sub.1, at which the branch pipe 2 is jointed to the main pipe 1 at an obtuse angle, so that deformations as shown in FIGS. 2, 3 result, unless the rigidity of the main pipe 1 is high enough.
To cope with this, reinforcements as shown in FIGS. 4 to 9 have been hitherto used for pipe joint constructions 3A, 3B, 3C.
FIGS. 4 and 5 show reinforcements by means of a gusset plate 4 which is welded along the generation line of the main pipe 1 for reinforcing the axial rigidity of the main pipe 1.
FIGS. 6 and 7 show reinforcements by means of ring stiffener 5 which are secured on the outer surface of the main pipe 1, thereby reinforcing the peripheral rigidity of the main pipe 1.
FIGS. 8 and 9 show reinforcement by means of "can", that is, a cylindrical shell 6 is secured on the outer surface of the main pipe 1, to improve the rigidity thereof both in the axial and in the circumferential directions of the main pipe 1.
However, the gusset type reinforcement suffers from a drawback in that stresses tend to concentrate at the point C.sub.4 on the outer surface of the branch pipe 2, resulting in marked lowering in fatigue strength of the branch pipe 2.
Meanwhile, the ring stiffener type reinforcement poses a problem of an inconvenient operation, while the can type reinforcement increase in weight of the pipe joint construction 3C, itself.